Effects of Increasing Austenitizing Temperature 2

[alwilliams]

In hardening a 1.1% C drill rod, if you increase the austenitizing T from 800 C to 900 C, why would you get an undesirable increase in grain size and increased C content of the austenite? Also, with higher C austenite, why would that increase the amount of retained austenite, decrease the attainable hardness, and increase the possibility of quench cracks?

 

[metman]

Once you elevate the temperature above the recrystallliztion temperature any further increase in temperature will cause grain growth. Typically larger grain size is undesirable compared to smaller grain size because toughness is dependant to a large degree on grain size. Finer grain size achieves greater toughness whereas larger grain size relates to more brittleness.

As temperature is increased near the upper critical temperature, more C is soluble in Austenite. It requires an understanding of the Iron-Iron Carbide diagram to grasp this. I do not completely understand the mechanism of retained austenite but will guess that higher C austenite will lower the Ms (Martensite start temperature) and/or the Mf (Martensite finish temperature) thereby producing more retained austenite at a given quench temperature.

This should not decrease the attainable hardness since larger grain size increases attainable hardness. However, more retained austenite means lower hardness. On the other hand, retained austenite can be transformed to Martensite by cooling to below the Mf after the intitial quenching process.

The increased possibilty of quench cracks is because there is higher attainable martensite percentage with higher C austenite if quenched at an initial quench temperatrure which is too low which in this instance probably about 200F.

Jesus is THE life,
Leonard

 

[metengr]

Question 1; Grain growth in metals basically follows the same principles or reaction kinetics as diffusion, where grain size is proportional to the square root of time * a grain growth coefficient. This grain growth coefficient is similar to a diffusivity coefficient, which is proportional to temperature. So, the higher the temperature for the same amount of time, results in grain growth.

Question 2; Referring to the iron-carbon phase diagram, the solubility of carbon increases as a function of increasing temperature. This is due to the ability of the austenite phase, which is a face centered cubic structure, to accommodate more carbon atoms at higher temperature. Also, the diffusivity of carbon atoms is much higher than other elements, so carbon atoms can rapidly enter and dissolve in austenite.

Question 3; Carbon is an austenite former, so as the carbon content increases, the ability to retain austenite increases at lower temperatures. The retained austenite is what reduces hardness.

Quench cracks are normally formed under conditions where the surface of a part cools more rapidly than the core. For example, if we take a steel bar (like an AISI Type 1080) and quench in water, the outer region of the bar will immediately transform to martensite. While the bar is still cooling, the core of the bar begins to transform to other phases at lower temperatures. During the formation of these other phases in the core, transformation stresses are produced because of differences in volume between the martensite and the other phases. If the martensite is too brittle and can't accommodate the transformation stresses from the core, the martensite will either deform or crack. Because carbon effects the hardness of martensite (as carbon content increases the hardness of martensite also increases), higher carbon alloys will generally have a greater tendency to develop quench cracks. This is the reason why certain alloying elements are added to steels to slow down the rate of phase transformation (lower the tranformation temperature), enabling the outer surface and core of the bar to transform together.

 

[alwilliams]

Metman, How did you know 900 C was above the recrystallization T?

 

[metman]

alwilliams,
The lower critical temperature where phase change occurs from BCC to FCC corresponds to the Rx temp. In this case 1333F as I recalll. Also this above .35X3257R = 1139F for Iron but Steel has a lower melting point than Iron.

Jesus is THE life,
Leonard